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12-06-2007, 06:00 PM   #1
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Auto focus, aperture and focal length

Greetings

How this three parameters work together ?

I'm considering to purchase one of the following; DA 70 or FA 77 or DFA 100 , which of them will give me a fastest focus in low light conditions (to freeze motion) while using with a K100D ?

TIA
andy

12-06-2007, 06:24 PM   #2
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The one with the lowest aperature, I think the 77 is a 1.9.
12-07-2007, 08:07 AM   #3
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Well, the widest maximum aperture (1.8 on the 77) will allow the most light for focusing, but as an all-metal lens, there is more mass for the AF motor to move. I guess it all depends on what you mean by "low light" -- no AF works well in very low light (i.e. a dim room at night).
12-07-2007, 08:33 AM   #4
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QuoteOriginally posted by xandy Quote
Greetings
How this three parameters work together ?
The main relationship in this context is that having a wider maximum aperture allows more light for the autofocus system, which should help it focus faster.

QuoteQuote:
I'm considering to purchase one of the following; DA 70 or FA 77 or DFA 100 , which of them will give me a fastest focus in low light conditions (to freeze motion) while using with a K100D ?
Normally when people talk about freezing motion, they mean a high shutter speed. A wider aperture will help with that. I think also, though, that you mean getting a focus lock quickly on moving subjects. The answer here is: the focus assist from the built-in flash (blinkyblinkyblinky!) will help a little bit, but in low light, this is hard for any autofocus system, and Pentax by all accounts isn't the fastest available. (Although haven't tested extensively, my K10D it seems slightly faster than my friend's old Canon 10D but much slower than his 30D.) In many situations, your best bet is still going to be to learn to manually focus quickly.

12-07-2007, 08:08 PM   #5
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Thank you All !
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